1. Field of the Invention
The present invention relates to a method of calculating optical frequency spectrum for use in an optical-spectrum measuring apparatus for measuring optical spectrum characteristics of a light source.
2. Description of the Related Art
FIG. 4 is a block diagram showing the structure of a conventional optical-spectrum measuring apparatus.
Referring to FIG. 4, reference numeral 10 designates a light source containing a variety of wavelength components and capable of emitting light, the spectrum of which is to be measured. Reference numeral 12 designates an incident slit for limiting the intensity of light emitted from the light source 10. Reference numeral 14 designates a concave mirror for converting light made incident on the concave mirror 14 through the incident slit 12 into parallel light beams.
Reference numeral 16 designates a diffraction grating having the surface provided with a multiplicity of grooves so as to, for each wavelength, spatially split the parallel light beams converted by the concave mirror 14. The diffraction grating 16 is mounted on a stage 17 which is rotative in a direction indicated with symbol D1 so as to be rotated in the direction indicated with the symbol D1 when the stage 17 is rotated. Reference numeral 18 designates a concave mirror for focusing only a light beam made incident on the concave mirror 18 to the position of an emission slit 20, the light beam being included in the light beams split by the diffraction grating 16 for each wavelength. Reference numeral 20 designates the emission slit for limiting bandwidth of the wavelengths of the light beam focused to the position of the slit by the concave mirror 18.
The incident slit 12, the concave mirror 14, the diffraction grating 16, the concave mirror 18 and the emission slit 20 constitute a Czerny-Turner spectroscope.
Reference numeral 22 designates a photodetector, such as a photodiode, to convert the intensity of light emitted from the emission slit 20 into an electric signal. Reference numeral 24 designates an amplifier for amplifying the electric signal output from the photodetector 22. Reference numeral 26 designates an analog/digital converter (hereinafter referred to as an D/A converter) for converting the value amplified by the amplifier 24 into a digital signal.
In the drawing, reference numeral 28 designates a motor for rotating the stage 17 on which the diffraction grating 16 is mounted. When a rotating shaft 29 of the motor 28 is rotated in a direction indicated with symbol D2, the stage 17 and the diffraction grating 16 are rotated in the direction indicated with the symbol D1. Reference numeral 30 designates a motor rotating circuit for controlling the rotation of the rotating shaft 29 of the motor 28 in response to a control signal output from a CPU 34 to be described later.
Reference numeral 32 designates a slit-width control unit for changing the width of the emission slit 20 in response to a control signal output from the CPU 34 to be described later.
Reference numeral 36 designates a display unit, such as a CRT (Cathode Ray Tube) or a liquid crystal display unit. The CPU 34 is connected to the A/D converter 26, the motor rotating circuit 30, the slit-width control unit 32 and the display unit 36 through a bus B. The CPU 34 outputs a control signal for controlling each of the motor rotating circuit 30 and the slit-width control unit 32. Moreover, the CPU 34 calculates the digital signal output from the A/D converter 26 so as to display, for example, the spectrum distribution on the display unit 36.
In the foregoing structure, light emitted from the light source 10 is made incident on the incident slit 12. Light allowed to pass through the incident slit 12 is converted into parallel light beams by the concave mirror 14 so as to be made incident on the diffraction grating 16. The diffraction grating 16 is rotated by the motor 28 around a shaft which is in parallel with the many grooves formed thereon so as to make an arbitrary angle from the parallel light beams. The arbitrary angle is determined when the motor rotating circuit 30 controls the motor 28 in response to the control signal output from the CPU 34.
The diffraction grating 16 spatially splits incident parallel light beams for each wavelength. Among the wavelengths spatially split by the diffraction grating 16, only light having a wavelength determined by an angle made between a direction of transmission of the parallel light beams and the diffraction grating 16 is emitted to the concave mirror 18. The concave mirror 18 focuses only light having the wavelength, which has been made incident on the concave mirror 18, to the position of the slit of the emission slit 20. Only a wavelength component within the width of the emission slit 20 is allowed to pass through the emission slit 20. The slit-width control unit 32 sets the width of the emission slit 20 in response to the control signal output from the CPU 34.
The photodetector 22 receives light allowed to pass through the emission slit 20 to convert the light into an electric signal proportional to the intensity of the light. The amplifier 24 amplifies an output from the photodetector 22 to a voltage suitable to be input to the A/D converter 26. The A/D converter 26 converts an output from the amplifier 24 into a digital signal. The digital signal output from the A/D converter 26 is supplied to the CPU 34. The CPU 34 calculates the digital signal. The CPU 34 outputs a result (for example, spectrum distribution) of a calculation to the display unit 36 through the bus B. The display unit 36 displays contents in accordance with the result of the calculation output from the CPU 34.
The procedure of the measurement will now be described. The CPU 34 outputs a control signal to the slit-width control unit 32 so as to set the width of the emission slit 20. Then, the CPU 34 issues a command to the motor rotating circuit 30 to change the angle of the diffraction grating 16 so as to set a wavelength which is allowed to pass through the emission slit 20. Moreover, the CPU 34 fetches the intensity of light allowed to pass through the emission slit 20 from the output of the A/D converter 26. The CPU 34 outputs a control signal to the motor rotating circuit 30. Thus, the wavelength allowed to pass through the emission slit 20 is swept from a measurement-start wavelength to a measurement-completion wavelength. Characteristics about the relationship between the measuring wavelength and the intensity of light obtained repeatedly are displayed on the display unit 36.
Recently, in the field of the optical communication, an optical spectrum is usually displayed as an optical frequency spectrum, that is, a characteristic of the relationship between an optical frequency and the intensity of light in place of display as a characteristic of the relationship between a wavelength and the intensity of light. In the above case, the measuring wavelength at each of the measuring points is converted into an optical frequency in accordance with the characteristics about the relationship between the measuring wavelength and the intensity of light which have repeatedly been obtained. The optical frequency is, as an optical frequency spectrum, displayed on the display unit 36. Recently, some of optical spectrum measuring apparatuses put on the market have a function capable of selectively displaying an optical spectrum and an optical frequency spectrum.
Incidentally, the bandwidth RB (also called a wavelength resolution) allowed to pass through the spectrometer 5 of the czerny-Turner spectroscope type shown in FIG. 4 is substantially expressed by the following equation (1). Note that the following expression is satisfied under the conditions that the focal distance of the concave mirror 14 is the same as that of the concave mirror 18 and the width of the slit of the emission slit 20 is larger than that of the incident slit 12. ##EQU1##
In the above equation, d is intervals among the grooves provided for the diffraction grating 16, m is the number of order of diffractions, f is the focal distance of each of the concave mirrors 14 and 18, S is the width of the slit of the emission slit 20 and .beta. is the angle made between a direction of light emitted to the concave mirror 18 which is included in light diffracted by the diffraction grating 16 and the normal of the diffraction grating 16.
The bandwidth of the wavelengths of the spectrometer 5 of the optical spectrum measuring apparatus must appropriately be set in accordance with the type of the light source when the measurement is performed. According to equation (1), the width of the emission slit 20 must be changed to change the bandwidth RB of the spectrometer 5. To arbitrarily set the bandwidth of the wavelengths of the spectrometer 5, the conventional technique has been arranged such that the width of the slit of the emission slit 20 is mechanically controlled by the slit-width control unit 32. Therefore, the structure is complicated excessively and a troublesome adjustment operation must be performed.
When the measuring wavelength is changed, the diffraction grating 16 must be rotated in the direction indicated with the symbol D1. When the angle of the diffraction grating 16 made from the parallel light beams is changed, also the angle .beta. in equation (1) is changed. According to equation (1), change in the bandwidth of the wavelengths of the spectrometer 5 depends on the measuring wavelength. Therefore, the characteristic of the bandwidth of the wavelengths of the spectrometer 5 is, for example, as shown in FIG. 5. FIG. 5 is a graph showing an example of the characteristic of the spectrometer 5 shown in FIG. 4.
Referring to FIG. 5, a curve given symbol G1 indicates an example of the characteristic of the bandwidth of the wavelengths of the spectrometer 5 shown in FIG. 4 while a curve given symbol G2 indicates an example of the characteristic of the bandwidth of the optical frequencies.
In general, the optical spectrum of light to be measured is wider than the bandwidth of the wavelengths of the spectrometer 5. Therefore, if the spectrometer 5 has the bandwidth of the wavelengths having the characteristic shown in FIG. 5, the measured optical spectrum has a characteristic that the portion having a short wavelength is raised. Thus, there arises a problem that an accurate optical spectrum cannot be obtained.
Also when the optical spectrum is displayed as the characteristic about the relationship between the optical frequency and the intensity of light, a similar problem arises because change in the bandwidth of the optical frequencies of the spectrometer 5 depends on the measuring wavelength or the measuring optical frequency.
When, for example, the characteristic of the bandwidth of the wavelengths of the spectrometer 5 indicated with symbol G1 is converted into the characteristic of the bandwidth of the optical frequencies, a characteristic indicated by a curve indicated with symbol G2 is obtained. As compared with the bandwidth of the wavelengths, greater change occurs.
The conventional technique has the arrangement that the measured characteristic about the relationship between the wavelength and the intensity of light is used to simply convert the wavelength into the optical frequency so as to be used as the optical spectrum. Therefore, the foregoing problem cannot be overcome.